1. Field of the Invention
The invention relates to a bandgap circuit and start circuit thereof. Particularly, the invention relates to a bandgap circuit and start circuit thereof with start function.
2. Description of Related Art
FIG. 1 is a circuit diagram of a conventional bandgap circuit. As shown in FIG. 1, the bandgap circuit 100 includes a start circuit 110 and a reference current generating circuit 120. The reference current generating circuit 120 includes a plurality of current mirrors 121-124, and the current mirrors 121-124 are connected in cascade with each other and have bias nodes N11-N14. Moreover, the cascade current mirrors 121-124 are electrically connected to ground through bipolar transistors BT11 and BT12 and a resistor R1. In this way, the reference current generating circuit 120 can map a bias current IB1 proportional to absolute temperature (PTAT) through P-channel transistors MT11 and MT12.
To guarantee the reference current generating circuit 120 to normally provide the bias current IB1, the start circuit 110 is used to break the reference current generating circuit 120 away from a zero-current state. During an operation, one of bias voltages VB11 and VB12 on the bias nodes N13 and N14 is transmitted to the start circuit 110, and the start circuit 110 determines whether or not to provide a start voltage VT1 to the bias node N11 or N12 according to a conducting state of an N-channel transistor MN 12. For example, FIG. 2 is a timing diagram of a power voltage. Referring to FIG. 2, according to a power on sequence, the start circuit 110 respectively starts the reference current generating circuit 120 during time intervals T21 and T22.
During the time interval T21, a power voltage VD1 is gradually increased from the lowest level (for example, 0 volt) to a level LV21. Moreover, during an initial increasing stage of the power voltage VD1, the bias voltages VB11 and VB12 approach to the lowest level, so that an N-channel transistor MN11 cannot be turned on. Meanwhile, a gate voltage of the N-channel transistor MN12 is pulled up to a high voltage level, so that the N-channel transistor MN12 is turned on. In this way, the start circuit 110 can output the start voltage VT1, so as to break the reference current generating circuit 120 away from the zero-current state. Thereafter, the bias voltages VB11 and VB12 are increased as the power voltage VD1 increases, so that the N-channel transistor MN11 is turned on. Now, the gate voltage of the N-channel transistor MN12 is pulled down to the low voltage level, so that the N-channel transistor MN12 cannot be turned on. In this way, the start circuit 110 stops outputting the start voltage VT1, and the reference current generating circuit 120 can normally supply the bias current IB1.
However, when the start circuit 110 performs the start operation for a second time, i.e. during the time interval T22, the power voltage VD1 is gradually increased from a level LV22 to the level LV21. Now, since the power voltage VD1 is not completely pulled down to the lowest level (for example, 0 volt), the bias voltages VB11 and VB12 cannot be completely discharged. Therefore, during the initial increasing stage of the power voltage VD1, the transistor MN1 is kept in the turn-on station, so that the transistor MN12 cannot be turned on, and the current mirrors 121-124 cannot produce an initial current. Therefore, the reference current generating circuit 120 cannot be broken away from the zero-current state.
In other words, when the power voltage is not completely pulled down to the lowest level, or when the power voltage is turned off and is quickly turned on again, since the bias voltages on the bias nodes are not completely discharged, the conventional start circuit 110 may miss-judge a time point of the starting operation. In other words, the conventional start circuit 110 has a chance to fail starting the reference current generating circuit 120 in some power on sequence.